Image sensors that are used for cameras in automobiles need to have a dynamic range more than 5 times greater than standard image sensors. This is because direct or reflected sunlight obscures detail in shadows. To image detail in shadows and highlight conditions, several different methods have been used. One method, such as U.S. Pat. No. 3,919,587, uses pixels with photodiodes having a non-linear photo-response. In another method, U.S. Pat. No. 6,777,661 uses pixels having two photosensitive regions with high light and low light photo-response. Finally, U.S. Pat. No. 6,069,377 uses a timer to measure the time it takes to reach saturation and reads out this time out along with the measured charge.
The disadvantage of the prior art is that reflected sunlight, especially from a wet road surface, can be so bright that image sensors with more than 10 times normal dynamic range are still not sufficient to image objects in reflected sunlight and shadows.
FIG. 1 illustrates light being reflected from a road surface. The electric field vector of the light incident upon the road surface is randomly polarized. The light has approximately equal electric fields that are parallel and perpendicular to the road plane of incidence. After reflection, the light has strong polarization with the electric field vector parallel to the plane of incidence (the road surface). It is noted that sunglasses are often designed with a polarizer to filter out light with the electric field vector parallel to the road surface.
A camera could place a polarizer in front of the image sensor, but this is undesirable because of the extra cost, and it removes 50% of the light from low light shadows.
The present invention provides an image sensor capable of selectively rejecting sunlight reflected from road surfaces while allowing light from other objects to be imaged. The present invention also integrates the polarizer with the image sensor.